internal/testutil: Add functions to dump printable chars

internal/testutil/testutil.go

@@ -97,6 +97,32 @@ func (sn Snake) String() string {
 	return out.String()
 }
 
+func (sn Snake) CharsString() string {
+	if len(sn) == 0 || len(sn[0]) == 0 || len(sn[0][0]) == 0 {
+		return "EMPTY"
+	}
+
+	out := strings.Builder { }
+	for index, sector := range sn {
+		if index > 0 { out.WriteString(" : ") }
+		out.WriteRune('[')
+		for index, variation := range sector {
+			if index > 0 { out.WriteString(" / ") }
+			for _, byt := range variation {
+				run := rune(byt)
+				if unicode.IsPrint(run) && run < 0x7F {
+					out.WriteRune(run)
+				} else {
+					out.WriteRune('.')
+				}
+				out.WriteRune(' ')
+			}
+		}
+		out.WriteRune(']')
+	}
+	return out.String()
+}
+
 // HexBytes formats bytes into a hexadecimal string.
 func HexBytes(data []byte) string {
 	if len(data) == 0 { return "EMPTY" }